Stereodon circinalis (Hooker) Mitten
Plants light gray- to golden-green or dark green, 3--5(--10 or more) cm. Stems slender, irregularly to regularly pinnate (occasionally 2-pinnate) or irregularly branched, usually creeping and firmly affixed by rhizoids, reddish brown, lacking hyalodermis or central strand; pseudoparaphyllia filamentous to lanceolate, usually terminated by an elongate cell or 1-seriate tip 2--4 cells long. Leaves of stem falcate-secund to circinate, ovate- to triangular-lanceolate, often asymmetric at base with one side, somewhat to strongly auriculate, slightly decurrent, narrowing to a long acumination, 1.5--2.2 × 0.5--0.7 mm, margins serrulate, plane, rarely slightly recurved on one side, costae indistinct; branch leaves smaller, 1.1--1.5 × 0.3--0.4 mm, more strongly serrulate, median cells 60--80(--100) × 4--5 µm, with thin or thick porose walls, basal cells broader with porose walls, golden yellow, alar cells few, subquadrate to rounded-triangular, 2--5 in marginal row, proximal ones inflated, reddish brown, sometimes hyaline on margin. Sexual condition dioicous; antheridial plants similar to archegonial or phyllodioicous with dwarf males epiphytic on archegonial plants; perichaetial leaves erect, oblong-lanceolate, inner erect, the outer recurved, slender point serrulate, not plicate. Seta smooth, reddish brown, 0.6--1.5(--2) cm. Capsule reddish brown, oblique to horizontal, ovoid to ovoid-oblong, 0.8--1.5 × 0.5--0.7 mm, operculum conic-apiculate, annulus 1--2 seriate; endostomial cilia 1--2.
Sporophytes produced autumn--winter (Sept.--Dec.), capsules usually mature Jan.--Feb. Lowland to subalpine coniferous forests, commonly epiphytic on tree trunks, also on decaying logs and rock; 0--1500 m.; endemic to western North America; Alta., B.C.; se Alaska, Calif., Idaho, Mont., Oreg., Wash.
The often asymmetric stem leaves bearing a long-attenuate serrulate point, with one side auriculate with usually colored alar cells, the small sporangia that mature in early spring make Hypnum circinale an easy species to recognize. Its closest affinities are with the east Asian H. tristoviride (Brotherus) Paris, which it strongly resembles in vegetative characters. In eastern North America H. andoi resembles some forms of H. circinale in size and appearance, but the leaf bases, especially the nature of the alar cells as well as auriculation in H. circinale (absent in H. andoi) are reliable distinguishing features.